Fix abstract classes with deferred bindings (issue #41)

[krystophny]

Summary

• Generate Fortran wrappers and Python methods for deferred bindings on abstract types
• Enable polymorphic dispatch through the base class when factory functions return class(base_type)

Changes

• transform.py: Resolve deferred binding prototypes to abstract interface procedures, marking them with 'deferred' and 'method' attributes
• LinkBoundDType: Handle resolved deferred bindings (skip raw Prototypes)
• Re-enable issue41_abstract_classes example in test suite

Problem

When a factory function returned a polymorphic class(myclass_t) object, Python wrapped it as myclass_t which didn't have deferred methods like get_value. This caused:

AttributeError: 'myclass_t' object has no attribute 'get_value'

Solution

Generate Fortran wrapper code for deferred bindings that uses class() for polymorphic dispatch:

subroutine f90wrap_myclass_base__get_value__binding__myclass_t(self, value)
    type myclass_t_wrapper_type
        class(myclass_t), allocatable :: obj
    end type
    ...
    call self_ptr%p%obj%get_value(value=value)  ! polymorphic dispatch
end subroutine

The Python wrapper method on the abstract base class calls this Fortran wrapper, which dispatches to the correct implementation via Fortran's polymorphism.

Test plan

• issue41_abstract_classes example now passes
• fortran_oo example still passes (tests OO features)
• extends example still passes (tests inheritance)

Stacked on #322